Dockboards or dock levelers are adapted for installation on loading docks to span the gap between the dock and the bed of a truck or other carrier when that truck is in position to be loaded or unloaded. These devices have found wide utilization in the materials handling industry. These devices have a ramp or deck which is hingedly mounted at its rear edge to the dock. The devices may be either pit or box mounted. Typically. the entire leveler structure is positioned within a shallow pit so that the rear hinged edge of the deck is flushed with the top surface of the dock. The deck is thus maintained in a flush position with the dock when not in use. This is the so-called cross-traffic position.
When in use. the deck is first angularly inclined upwardly relative to the dock to accommodate trucks having beds at different heights. To use the dockboard the truck to be loaded or unloaded is thus backed into position adjacent the front of the dockboard and the rear edge of the bed is adjacent but is spaced slightly from the dock. A hold-down device on the dockboard is then released so that the deck will raise upwardly, it being generally biased in an upward position by means of a spring. The upward swing of the deck causes the extension lip to be raised after it has cleared the rear end of the truck bed. With the deck now in a raised position and the lip extended. the operator walks onto the deck and by his weight lowers it into position. This causes the extension lip to move downward into engagement with the bed and thus. the lip and the front part of the deck are supported by the bed of the truck. With this technique the ramp, together with the extension lip, bridges the gap between the dock and the truck bed. The dockboard provides this bridge even though the bed on the truck may be above or below dock level. Additionally, since the truck tends to float, that is, move up or down depending on the degree of loading or unloading, the deck and the lip will automatically follow these variations in height so that repositioning is not needed.
The art is replete with the number of examples of such devices. Typical are U.S. Pat. Nos. 3,137,017, 3,995,342, 4,014,059, 4,279,050, 4,328,602, and 4,619,008. These are patents are samples and representative of an entire class of devices which have been the subject of extensive technological development.
One of the difficulties with dockboards is providing sufficient force to have the lip automatically extend as the dockboard raises. While a number of complicated spring and lever schemes have been attempted, they add to the complexity of the device and its overall cost. While additional stored energy can be provided by utilizing-additional springs to thereby provide sufficient biasing force, more powerful springs make it difficult to lower or "walk down" the dockboard when in its raised position. Consequently, there exists in the art a need to provide a simple mechanism which assists the raising the lip without unduly complicating the mechanical assembly. In prior art devices, the lip is generally extended by means of a linkage such as a chain which is attached to the lower frame. The chain is tightened as the deck reaches the top of its travel thereby causing the lip to be rotated outward. To assist in this rotation the lip is generally partially counterbalanced so that, it may be easily extended. However, it must not be so nearly counterbalanced that it will fail to fall by gravity to the stored positioned.
In the prior art, the technique of counterbalancing usually involves a spring mechanism connected between the deck and an arm fastened to a lip plate. While satisfactory for most installations, a problem occurs in that a lip, which is properly spring counterbalanced at dock level, will be inadequately balanced when the dock leveler is raised to a higher angle. This is because the moment arm from the lip hinge to the center of gravity of the lip is different in the level position than it is in a raised position. Thus, if the counterbalance allows the lip to fall when the deck is level, then the lip can only be partially counterbalanced when the deck is raised. Thus, the dock leveler spring force must be increased so that the deck is raised with greater force in order to force the lip to extend. This, in turn, requires a greater force to walk the leveler down to the stored position.